Axial Flow Fan

ABSTRACT

An axial flow fan includes a motor placed on a frame and including a rotor rotatable around a rotation axis. An impeller is attached to an outer circumference of the rotor to rotate around the rotation axis and includes blades for generating an air flow when the rotor rotates. A housing surrounds the impeller to form a passage for the air flow. Ribs extend from the frame to the housing, thereby securing the frame to the housing. Each rib includes an air guide face which faces the impeller. An angle of average inclination of the air guide face with respect to the rotation axis decreases in a direction away from the rotation axis. The average inclination of the air guide face is defined as inclination of a straight line approximately connecting both ends of the air guide face on a plane perpendicular to a radial direction.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to an axial flow fan, and more particularly, relates to a shape of ribs in the axial flow fan.

2. Description of the Related Art

At present, electronic devices are provided with many cooling fans for radiating a heat generated in the electronic devices. The generated heat amount has been increasing with enhancement of the performance of the electronic devices, and therefore a required cooling performance of fans has become higher. In order to improve the cooling performance of fans, it is necessary to improve flow rate characteristics and static pressure characteristics of the fans. The improvement of both the characteristics requires the fans to rotate at high speeds. On the other hand, demands for reduction of noises in many electronic devices have increased with increase in the use of the electronic devices at home or offices.

Usual fans include a motor, an impeller having a plurality of blades attached to a rotor of the motor, and a housing which supports a stator of the motor and surrounds an outer circumference of the impeller. The housing includes a cavity which forms a passage for an air flow generated by rotation of the impeller, a frame which supports the stator, and a plurality of ribs which connect the cavity and the frame to each other. The ribs are arranged to cross the passage. Thus, windage loss at the ribs causes energy loss, lowering both a flow rate and a static pressure of the air flow. Moreover, the air flow interferes with the ribs to cause an interference noise which is one noise source in the fans.

In order to overcome the above problems, a rib having a streamlined cross section has been proposed. The rib is arranged in such a manner that its principal axis of its cross-sectional shape is parallel to the air flow.

In order to actually design the proposed rib, however, it is necessary to measure a direction of the air flow generated by rotation of the impeller. This direction is changed not only by the shape of the impeller but also by the revolution speed of the impeller. In addition, the shape of the cavity of the housing, the surface condition of the impeller, a state of placement of the fan in an electronic device, a temperature, and a humidity can also change the direction of the airflow. Since the direction of the airflow is changed with a small change in surroundings as described above, the proposed shape of the ribs can be designed only for a specific fan structure, a specific revolution speed, a specific condition of use, and the like. However, fans can have various structures, operate at various revolution speeds, and be used under various conditions. Considering those, the shape of the rib has to be designed.

BRIEF SUMMARY OF THE INVENTION

In order to overcome the above problems, it is an object of the present invention to provide a shape of ribs of a fan, which can improve an air-blowing performance without degrading noise characteristics, irrespective of a structure and a condition of use of the fan.

According to an aspect of the present invention, an axial flow fan includes: a motor including a rotor rotatable around a rotation axis; an impeller attached to an outer circumference of the rotor to rotate around the rotation axis together with the rotor, the impeller including a plurality of blades generating an air flow when the rotor rotates; a housing surrounding an outer circumference of the impeller to form a passage for the air flow; a frame on which the motor is placed; and a plurality of ribs approximately radiating from the frame and securing the frame to the housing. Each of the ribs includes an air guide face facing the impeller. The air guide face is an approximately flat face or a curved face having average inclination with respect to an axial direction parallel to the rotation axis. The average inclination is defined as inclination of a straight line approximately connecting both ends of the air guide face on a plane perpendicular to a radial direction perpendicular to the axial direction at a position in the radial direction. An angle of the average inclination becomes smaller in a direction away from the rotation axis.

A cross-sectional area of each of the ribs when seen in a longitudinal direction of that rib may be approximately constant at any position in a radial direction.

The angle of the average inclination of the air guide face of each of the ribs with respect to average inclination of one of the blades, which is located at a closest position to that rib in the axial direction, on a plane perpendicular to the radial direction may be approximately constant at any position in the radial direction. The average inclination of each of the blades is defined as inclination of a straight line connecting both ends of the blade on the plane perpendicular to the radial direction.

The angle of average inclination of the air guide face of each of the ribs with respect to average inclination of one of the blades, which is located at a closest position to that rib in the axial direction, on a plane perpendicular to the radial direction may be 100° or less at any position in the radial direction. The average inclination of each of the blades is defined as inclination of a straight line approximately connecting both ends of the blade on the plane perpendicular to the radial direction.

The angle of average inclination of the air guide face of each of the ribs with respect to inclination of a trailing edge of one of the blades, which is located at a closest position to that rib in the axial direction, on a plane perpendicular to the radial direction may be 100° or less at any position in the radial direction.

A cross-sectional shape of each of the ribs seen in the radial direction may be different at different positions in the radial direction.

Each of the ribs may be arranged at an angle to the radial direction, when seen in the axial direction.

The ribs may be curved toward one of a rotating direction of the impeller and a direction opposite to the rotating direction.

Each of the ribs may include a bottom face which is approximately parallel to a lower face of the housing and arranged in the same plane as the lower face of the housing.

Other features, elements, advantages and characteristics of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an axial flow fan according to an embodiment of the present invention.

FIG. 2 is a perspective view of a housing of the axial flow fan of FIG. 1.

FIGS. 3A to 3C show cross-sectional shapes of a rib and a blade seen at a given position in a radial direction.

FIG. 4 is a plan view of the axial flow fan of FIG. 1.

FIG. 5 is a plan view of a modified example of the axial flow fan of the embodiment of FIG. 1.

FIG. 6 is a plan view of the housing of the axial flow fan of FIG. 1.

FIGS. 7A to 7C are cross-sectional views of an exemplary rib according to the present invention, taken along line A-A, B-B, and C-C in FIG. 6, respectively.

FIGS. 8A to 8C are cross-sectional views of another exemplary rib according to the present invention, taken along line A-A, B-B, and C-C in FIG. 6, respectively.

FIGS. 9A to 9C are cross-sectional views of still another exemplary rib according to the present invention, taken along line A-A, B-B, and C-C in FIG. 6, respectively.

FIGS. 10A to 10C are cross-sectional views of further another exemplary rib according to the present invention, taken along line A-A, B-B, and C-C in FIG. 6, respectively.

FIGS. 11A to 11C are cross-sectional views of further another exemplary rib according to the present invention, taken along line A-A, B-B, and C-C in FIG. 6, respectively.

FIGS. 12A to 12C are cross-sectional views of further another exemplary rib according to the present invention, taken along line A-A, B-B, and C-C in FIG. 6, respectively.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 through 12C, preferred embodiments of the present invention will be described in detail. It should be noted that in the explanation of the present invention, when positional relationships among and orientations of the different components are described as being up/down or left/right, ultimately positional relationships and orientations that are in the drawings are indicated; positional relationships among and orientations of the components once having been assembled into an actual device are not indicated. Meanwhile, in the following description, an axial direction indicates a direction parallel to a rotation axis, and a radial direction indicates a direction perpendicular to the rotation axis.

FIG. 1 is a cross-sectional view of an axial flow fan according to an exemplary embodiment of the present invention. FIG. 2 is a perspective view of a housing of the axial flow fan of FIG. 1. FIG. 3A shows a cross-sectional shape of an impeller's blade seen in a radial direction, together with a rotating direction of an impeller and an air flow. FIGS. 3B and 3C are cross-sectional views of a rib and an impeller's blade located at a closest position to that rib, when seen at a given position in the radial direction. FIG. 4 is a plan view of the axial flow fan of FIG. 1. FIG. 5 is a plan view of a modified example of the axial flow fan of FIG. 1.

An axial flow fan A includes a motor placed on a frame 12. The motor includes a rotor in which an approximately cylindrical rotor yoke 31 with a cover is included. The rotor yoke 31 is driven by a current supplied from the outside of the axial flow fan A to rotate. An impeller 2 having a plurality of blades 21 is attached to an outer circumference of the rotor, i.e., an outer circumferential surface of the rotor yoke 31, and can rotate together with the rotor yoke 31 when the rotor yoke 31 rotates. The rotor yoke 31 includes a shaft 32 which has an end fixed at a center of the rotor yoke 31 by fastening. The shaft 32 serves as a rotation axis.

At a center of the frame 12, an approximately cylindrical bearing housing 12 a having a bottom is formed. In the bearing housing 12 a, a radial bearing 34 is press-fitted and supported. The radial bearing 34 includes an insertion hole for the shaft 32. The shaft 32 is inserted into the insertion hole to be rotatable. The radial bearing 34 is an oil-retaining bearing formed of porous material such as sintered material, with lubricating oil contained therein. Since the radial bearing 34 contains the lubricating oil, the radial bearing 34 can rotatably support the shaft 32 via the lubricating oil. However, the radial bearing 34 is not limited to a sliding bearing which rotatably supports the shaft 32 via the lubricating oil as described above. Instead of the sliding bearing, a roller bearing such as a ball bearing may be used. The type of bearing to be used is chosen in an appropriate manner, considering the required performance and cost of the axial flow fan A.

The axial flow fan A also includes a stator 3 as a part of the motor. The stator 3 is supported on an outer circumference of the bearing housing 12 a. The stator 3 includes a stator core 35, a coil 37, an insulator 36, and a circuit board 38. The stator core 35 is surrounded by the insulator 36 formed of insulating material so that an upper and a lower ends of the stator core 35 and each tooth are insulated. The coil 37 is wound around the teeth with the insulator 36 interposed therebetween. The circuit board 38 which controls driving and rotation of the impeller 2 is arranged at a lower end of the stator 3. In the circuit board 38, electronic components (not shown) are mounted on a printed circuit board to form circuitry. An end of the coil 37 is electrically connected to the electronic components on the circuit board 38 which is bonded and fixed to a lower part of the insulator 36. When a current supplied from the outside of the axial flow fan A is made to flow through the coil 37 via the electronic components including an IC and a hole element, a magnetic field is generated around the stator core 35.

On an inner circumferential surface of the impeller 2, the rotor yoke 31 which can reduce leakage magnetic flux to the outside of the axial flow fan A is provided. Moreover, a rotor magnet 33 as a part of the motor, which is magnetized to achieve multipole magnet, is attached to an inner circumference of the rotor yoke 31 inside the impeller 2. The rotor magnet 33 and the stator core 35 are opposed in the radial direction by inserting the shaft 32 fixed by fastening to the center of the rotor yoke 31 into the radial bearing 34. When a current flows through the coil 37, a rotating torque is generated in the impeller 2 by interaction of the magnetic field generated by the stator core 35 and a magnetic field formed by the rotor magnet 33 magnetized to achieve multipole magnet, thereby rotating the impeller 2 around the shaft 32 as a rotation axis. A change in magnetic flux from the rotor magnet 33 which is rotating is detected by the hole element. Based on this detection, an output voltage is switched by a drive IC. In this manner, rotation of the impeller 2 is controlled to be stable. During the rotation of the impeller 2, the blades 21 push air downward, thus generating an air flow approximately along the axial direction.

The frame 12 on which the motor is placed is disposed to be opposed to the circuit board 38 in the axial direction and has a shape of an approximately circular disk having approximately the same diameter as an outer diameter of the circuit board 38. The frame 12 is secured to the housing 1 with four ribs 13 which extend from the frame to the housing 1. Please note that the number of the ribs 13 for securing the frame 12 to the housing 1 is not limited to four. Three or five ribs may be provided, for example. The housing 1 is formed to surround an outer circumference of the impeller 2 and includes a cavity 11 serving as a passage for an air flow generated by rotation of the impeller 2. Outer circumferential portions of an upper and a lower end faces of the housing 1 are formed in an approximately square frame. Flange portions 14 are formed at four corners of the square, respectively, which spread radially outward. Each flange portion 14 has an attachment hole 14 a formed therein. When the axial flow fan A is mounted on a device in which the axial flow fan A is to be used, an attaching component such as a screw is inserted into the attachment hole 14 a. The four ribs 13 are arranged at regular angular intervals in the circumferential direction.

When orthogonally projected onto a plane perpendicular to the axial direction, the blades 21 of the impeller 2 are inclined toward the rotating direction of the impeller 2 in the circumferential direction. A cross-sectional shape of each blade 21 seen in the radial direction is an arc-like shape curved toward the rotating direction of the impeller 2, as shown in FIG. 3B. A fan used for cooling the inside of an electronic device is usually chosen, considering system impedance in the electronic device (i.e., a relationship between a static pressure and a flow rate in the electronic device) and a flow rate and a static pressure of the fan. In many electronic devices, electronic components, a power source, and the like are concentrated in a narrow space and therefore the system impedance is high. When the system impedance is high, it is hard for fans having a low static pressure to generate a sufficient air flow. For this reason, fans used for cooling the inside of electronic devices are required to have a high static pressure. In order to make the static pressure higher, there is an approach in which an interval between adjacent blades 21 when seen in the axial direction is made smaller. This can be achieved by making an arc length of an arc-like portion in the cross-sectional shape of each blade 21 seen in the radial direction longer radially outward. In this case, however, an axial height (i.e., a height in the axial direction) of each blade 21 increases radially outward. By making a difference in the axial height between at a radially inner position and at a radially outer position smaller, an effective volume of a space occupied by the blades 21 in the cavity 11 (which is a product of an area of the blade 21 when seen in the axial direction and the axial height of the blade 21) increases. Thus, an axial flow fan A which is high in both a flow rate and a static pressure can be designed. This can be achieved by making inclination of the blade 21 with respect to the axial direction larger radially outward.

A cross-sectional shape of the rib 13 seen in the radial direction has an approximately triangular shape formed by a bottom face 131, an air guide face 132, and a side face 133 connecting the bottom face 131 and the air guide face 132 to each other, as shown in FIG. 7A. The bottom face 131 is substantially perpendicular to the axial direction, i.e., substantially parallel to a lower end face of the housing 1 and a lower end face of the frame 12, and forms the same plane as that formed by the lower end faces of the housing 1 and the frame 12. The air guide face 132 guides an air flow generated by rotation of the impeller 2 and is arranged at an angle with respect to the axial direction. Although the air guide face 132 formed by a flat face is shown in FIG. 7A, the air guide face 132 may be a curved face. In a case of a curved face, average inclination of the curved air guide face 132 is defined as inclination of a straight line approximately connecting both ends of the curved air guide face 132 on a cross section perpendicular to the radial direction, and an angle of the air guide face 132 with respect to the axial direction is represented by the thus defined average inclination.

Since the ribs 13 are arranged to cross the passage for the air flow, the ribs 13 have to have such a shape that energy loss in the air flow when the air flow passes by the ribs 13 is minimized. If the ribs 13 have a streamlined shape in which a cross-sectional shape of each rib 13 seen in the radial direction is parallel to the air flow, energy loss in the air flow caused by hitting of the air flow against the ribs 13 becomes smaller as the thickness of the ribs 13 decreases. In a case where the ribs 13 are thin, however, the axial height of the ribs 13 has to be increased in order to obtain a sufficient level of strength of the ribs 13. The ribs 13 which are high in the axial height are not preferable, because the ribs 13 are close to the blades 21 and a noise generated by interference of the air flow with the ribs 13 becomes loud. Such a loud interference noise makes a noise level higher. Based on the above consideration, in the present embodiment, the ribs 13 are formed to have an approximately triangular cross-sectional shape when seen in the radial direction. This cross-sectional shape can increase both the thickness and strength of the ribs 13 while suppressing energy loss in the air flow caused by the ribs 13.

The air flow generated by rotation of the impeller 2 flows along the air guide faces 132 of the ribs 13 when passing by the ribs 13, and flows out of the cavity 11 to the outside of the axial flow fan A. The blades 21 are inclined toward the rotating direction of the impeller 2 as described above. Average inclination of each blade 21 on a cross section perpendicular to the radial direction is defined as inclination of a straight line approximately connecting both ends of the blade 21 on that cross section. The air flow does not flow parallel to the axial direction. Instead, an angle of the air flow with respect to the axial direction depends on the average inclination of the blade 21 and the air flow is discharged at an angle of 90° or less with respect to the average inclination of the blade 21. However, this angle is changed by the cross-sectional shape of the blades 21, the shape of the cavity 11, the revolution speed of the impeller 2, and an outside temperature at which the axial flow fan A is used. The average inclination of each blade 21 is different at different positions in the radial direction. Therefore, the angle of the air flow from each blade 21 is different at different positions in the radial direction.

In order to design the shape of the ribs 13 that can reduce energy loss in the air flow caused by the ribs 13, it is necessary to change average inclination of the air guide face 132 of each rib 13 depending on a position in the radial direction. Please note that average inclination of the air guide face 132 of each rib 13 on a plane perpendicular to the radial direction is defined as inclination of a straight line connecting both ends of the air guide face 132. By changing the average inclination of the air guide face 132 of each rib 13 in accordance with the angle of the air flow flowing from the blades 21, the energy loss in the air flow can be minimized. Since the angle of the air flow flowing from the blade 21 changes depending on the cross-sectional shape of the blades 21, the shape of the cavity 11, the revolution speed of the impeller 2, and the outside temperature at which the axial flow fan A is used, according to the present invention, the shape of the ribs 13 is designed in such a manner that an angle of the average inclination of the air guide face 132 of each rib 13 with respect to average inclination of one of the blades 21 that is located at a closest position to that rib 13 in the axial direction is 100° or less, considering the above change. In the present embodiment, the angle of the average inclination of the air guide face 132 of each rib 13 with respect to the blade 21 located at a closest position to that rib 13 in the axial direction is set to 90°, as shown in FIG. 3B. Moreover, the energy loss in the air flow can be made constant at any position in the radial direction by designing the shape of the ribs 13 in such a manner that the angle of the average inclination of the air guide face 132 of each rib 13 with respect to the inclination of the blade 21 located at a closest position to that rib 13 in the axial direction is the same at any position in the radial direction. In particular, when the angle of the average inclination of the air guide face 132 of each rib 13 with respect to the inclination of the blade 21 located at a closest position to that rib 13 in the axial direction is set to 90°, the energy loss can be reduced. In a case where a curvature of a curved portion of a cross-sectional shape of each blade 21 seen in the radial direction is relatively small with respect to an arc length thereof, the angle of the air flow from that blade 21 does not depend on average inclination of that blade 21 on a cross section perpendicular to the radial direction but depends on an angle of a trailing edge 211 of the blade 21 with respect to the axial direction. In this case, the cross-sectional shape of the ribs 13 is designed based on the angle of the trailing edge 211 in place of the inclination of the blade 21.

FIG. 6 is a plan view of the housing in the present embodiment. FIGS. 7A to 12C are cross-sectional views of the rib 13, taken along line A-A, line B-B, and line C-C, respectively. In an example of FIGS. 7A to 7C, the bottom face 133 is always formed in the same plane as the lower end face of the frame 12. Moreover, the length of the bottom face 131 in the cross-sectional shape seen in the radial direction becomes shorter radially outward, and the height of the side face 133 in that cross-sectional shape becomes higher radially outward. That is, the cross-sectional shape of the ribs 13 in the example of FIGS. 7A to 7C changes in such a manner that an angle θ of the air guide face 132 with respect to the bottom face 131 gradually increases radially outward, i.e., an angle of the average inclination of the air guide face 132 with respect to the axial direction becomes smaller in a direction away from the rotation axis. In this example, the ribs 13 are designed have a constant cross-sectional area when seen in a longitudinal direction. With this design, concentration of stress does not occur even when a load is applied to the ribs 13, and lowering of the strength of the ribs 13 can be suppressed. In addition, strength is not high at a joint of the frame 12 and each rib 13. However, by forming the bottom faces 131 of the ribs 13 in the same plane as the lower end face of the frame 12, concentration of stress can be suppressed and therefore lowering of the strength at the joint of the frame 12 and each rib 13 can be suppressed. The same can be applied to a joint of the housing 1 and each rib 13. That is, lowering of the strength at the joint of the housing 1 and each rib 13 can be suppressed by forming the bottom face 131 of each rib 13 in the same plane as the lower end face of the housing 1.

FIGS. 8A to 12C illustrate modified examples of the cross-sectional shape of the ribs 13 in the present embodiment. In the example of FIGS. 7A to 7C, the side face 133 of each rib 13 is parallel to the axial direction, whereas in an example of FIGS. 8A to 8C the cross-sectional shape of the ribs 13 changes in such a manner that an angle of the side face 133 with respect to the bottom face 131 becomes smaller radially outward. Therefore, in the example of FIGS. 8A to 8C, inclination of the side face 133 becomes close to inclination of the air guide face 132, so that energy loss in the air flow can be suppressed. However, the thickness of the ribs 13 is thinner in the example of FIGS. 8A to 8C than in the example of FIGS. 7A to 7C. Therefore, the strength of each rib 13 in the example of FIGS. 8A to 8C is lower than in the example of FIGS. 7A to 7C. In an example of FIGS. 9A to 9C, the length of the bottom face 131 in the cross-sectional shape seen in the radial direction is kept constant, and an angle of the air guide face 132 with respect to the bottom face 131 is increased radially outward. That is, an angle of the average inclination of the air guide face 132 with respect to the axial direction becomes smaller in the direction away from the rotation axis. In this case also, energy loss in the air flow can be suppressed. Moreover, since the axial height of the ribs 13 is kept constant in the example of FIGS. 9A to 9C, a cross-sectional area of each rib 13 when seen in the longitudinal direction of that rib 13 can be made constant. In an example of FIGS. 10A to 10C, a corner connecting the bottom face 131 and the side face 133 to each other is rounded. With this cross-sectional shape, it is possible to prevent the air flow which is to flow along the ribs 13 from flowing away the ribs 13, thus suppressing generation of turbulence. In the present invention, the cross-sectional shape of each rib 13 seen in the radial direction is not limited to an approximately triangular shape. For example, the cross-sectional shape of each rib 13 seen in the radial direction may be a shape of a static vane shown in FIGS. 11A to 11C, or a shape with both longitudinal ends rounded, as shown in FIGS. 12A to 12C.

Assuming that the air flow flows approximately parallel to the air guide face 132, an angle of the side face 133 with respect to the air flow in FIG. 7C is smaller than in FIG. 7A. At a radially outer position, the arc length in the cross-sectional shape of the blade 21 seen in the radial direction is longer and a circumferential velocity of the blade 21 is larger, as compared with those at the radially inner position. Therefore, a flow rate of the air flow generated by the blades 21 is higher at the radially outer position than at the radially inner position. Thus, by making an angle of the side face 133 of each rib 13 with respect to the air flow on a plane perpendicular to the radial direction smaller at the radially outer position, energy loss in the air flow can be suppressed. At the radially inner position, even if the angle of the side face 133 of each rib 13 with respect to the air flow is large, an effect of the ribs 13 on energy loss in the air flow is small because a flow rate is low. Therefore, it is possible to provide an axial flow fan which can achieve a high flow rate.

By setting an angle of average inclination of the air guide face 132 of each rib 13 with respect to average inclination of a blade 21 located at a closest position to that rib 13 in the axial direction to 90° on a plane perpendicular to the radial direction, not only energy loss in the air flow but also an interference noise generated by hitting of the air flow against the ribs 13 can be reduced. If the trailing edges 211 of the blades 21 and the ribs 13 are arranged approximately parallel to each other during rotation of the blades 21, the air flow pushed by the blades 21 hits the ribs 13 at the same time and therefore the interference noise becomes large. In order to avoid this, each rib 13 is arranged at an angle to the radial direction in such a manner that a radially outer end thereof is located backward of a radially inner end thereof in the rotating direction of the impeller 2, when seen in the axial direction, as shown in FIG. 4. In this case, the ribs 13 and the blades 21 cannot be parallel to each other because the blades 21 are inclined toward the rotating direction of the impeller 2 in the present embodiment. Moreover, as shown in FIG. 5 as a modified example, the thus arranged ribs 13 may be curved toward a direction opposite to the rotating direction of the impeller 2. Also in this case, the interference noise can be suppressed. Alternatively, in a case where the blades 21 are inclined toward the direction opposite to the rotating direction of the impeller 2 when being orthogonally projected onto a plane perpendicular to the axial direction, each of the ribs 13 is arranged at an angle to the radial direction in such a manner that a radially outer end thereof is located forward of a radially inner end thereof in the rotating direction of the impeller 2.

In the present invention, when a cross-sectional shape of each rib 13 seen at a given position in the radial direction is determined, it is only necessary to change average inclination of the air guide face 132 of each rib 13 in accordance with average inclination of the blades 21. Therefore, the ribs 13 can be easily designed.

According to the present invention, energy loss in an air flow generated by rotation of an impeller, which is caused by ribs, can be minimized. In addition, reduction in a flow rate and a static pressure of the air flow can be suppressed. Moreover, it is also possible to suppress an interference noise generated when the air flow passes by the ribs.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

1. An axial flow fan comprising: a motor including a rotor rotatable around a rotation axis; an impeller attached to an outer circumference of the rotor to rotate around the rotation axis together with the rotor, the impeller including a plurality of blades generating an air flow when the rotor rotates; a housing surrounding an outer circumference of the impeller to form a passage for the air flow; a frame on which the motor is placed; and a plurality of ribs extending from the frame to the housing and securing the frame to the housing; wherein each of the ribs includes an air guide face facing the impeller, the air guide face being an approximately flat face or a curved face having average inclination with respect to an axial direction parallel to the rotation axis, the average inclination being defined as inclination of a straight line approximately connecting both ends of the air guide face on a plane perpendicular to a radial direction perpendicular to the axial direction at a position in the radial direction, an angle of the average inclination becoming smaller in a direction away from the rotation axis.
 2. The axial flow fan according to claim 1, wherein a cross-sectional area of each of the ribs when seen in a longitudinal direction of that rib is approximately constant at any position in a radial direction.
 3. The axial flow fan according to claim 1, wherein an angle between the average inclination of the air guide face of each of the ribs and average inclination of one of the blades, which is located at a closest position to that rib in the axial direction, on a plane perpendicular to the radial direction is approximately constant at any position in the radial direction, the average inclination of each of the blades being defined as inclination of a straight line approximately connecting both ends of the blade on the plane perpendicular to the radial direction.
 4. The axial flow fan according to claim 1, wherein an angle between the average inclination of the air guide face of each of the ribs and average inclination of one of the blades, which is located at a closest position to that rib in the axial direction, on a plane perpendicular to the radial direction is 100° or less at any position in the radial direction, the average inclination of each of the blades being defined as inclination of a straight line approximately connecting both ends of the blade on the plane perpendicular to the radial direction.
 5. The axial flow fan according to claim 1, wherein an angle between the average inclination of the air guide face of each of the ribs and inclination of a trailing edge of one of the blades, which is located at a closest position to that rib in the axial direction, on a plane perpendicular to the radial direction is 100° or less at any position in the radial direction.
 6. The axial flow fan according to claim 1, wherein a cross-sectional shape of each of the ribs seen in the radial direction is different at different positions in the radial direction.
 7. The axial flow fan according to claim 1, wherein each of the ribs is arranged at an angle to a radial direction, when seen in the axial direction.
 8. The axial flow fan according to claim 1, wherein the ribs are curved toward one of a rotating direction of the impeller and a direction opposite to the rotating direction.
 9. The axial flow fan according to claim 1, wherein each of the ribs includes a bottom face which is approximately parallel to a face of the housing and arranged in the same plane as the face of the housing. 